Update mdbook and remove our custom table-of-contents plugin

[anoadragon453]

We're currently using mdbook 0.4.17 in CI:

synapse/.github/workflows/docs.yaml

Lines 58 to 61 in a8e5c31

	- name: Setup mdbook
	uses: peaceiris/actions-mdbook@ee69d230fe19748b7abf22df32acaa93833fad08 # v2.0.0
	with:
	mdbook-version: '0.4.17'

synapse/.github/workflows/docs-pr.yaml

Lines 21 to 24 in a8e5c31

	- name: Setup mdbook
	uses: peaceiris/actions-mdbook@ee69d230fe19748b7abf22df32acaa93833fad08 # v2.0.0
	with:
	mdbook-version: '0.4.17'

mdbook 0.5 was released on November 17th (migration guide) and has support for admonitions (those little Note, Tip, Warning, etc. boxes) among other nice goodies. It also includes built-in support for headers for the current page to be displayed in the left sidecar.

The latter effectively replaces our hacked-together table-of-contents plugin, which would be nice to get rid of (and which currently only shows up on the page if you're zoomed out far enough, which is annoying).

Summary of tasks:

• Update mdbook to 0.5.x (0.5.1 is out currently)
• Remove our custom table-of-contents plugin
• Ensure our version picker plugin and other CSS changes still function